The present invention relates to hermetic compressors and more particularly to two stage rotary compressors using carbon dioxide as the working fluid.
Conventionally, multi-stage compressors are ones in which the compression of the refrigerant fluid from a low, suction pressure to a high, discharge pressure is accomplished in more than one compression process. The types of refrigerant generally used in refrigeration and air conditioning equipment include chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). Additionally, carbon dioxide may be used as the working fluid in refrigeration and air conditioning systems. By using carbon dioxide refrigerant, ozone depletion and global warming are nearly eliminated. Carbon dioxide is non-toxic, non-flammable, and has better heat transfer properties than CFCs and HCFCs, for example. The cost of carbon dioxide is significantly less than CFC and HCFC. Additionally, it is not necessary to recover or recycle carbon dioxide, which contributes to significant cost savings in training and equipment.
In a two-stage compressor, the suction pressure gas is first compressed to an intermediate pressure. The intermediate pressure gas is then generally collected in an accumulator. From the accumulator, the intermediate pressure gas is drawn into a second compressor mechanism where it is compressed to a higher, discharge pressure for use in the remainder of the refrigeration system.
The compression mechanisms of the two-stage compressor may be in one of two orientations. The compression mechanisms may be stacked adjacent one another on one side of the motor, or positioned with one compression mechanism located on opposite sides of the motor. When the compression mechanisms are located adjacent one another, on one side of the motor, problems may occur during compressor operation. Such problems may include overheating of the suction gas supplied to the first stage compression mechanism which affects volumetric efficiency of the compressor performance. Heat transfer from the discharge pressure pipe heats the incoming suction pressure gas due to the close proximity of the pipes. Further, overheating due to the closeness of the compression mechanisms may create problems including additional reduction of the compressor efficiency and possible reliability issues.
In general, the compressor motor is located within the compressor housing and is surrounded by either suction pressure gas, or cooled intermediate pressure gas, which cools the motor during compressor operation. The suction pressure gas or cooled intermediate pressure gas surrounding the motor is then supplied to the second stage compression mechanism. If the suction or cooled intermediate pressure gas is overheated as discussed above, the motor and the gas entering the second stage compression mechanism may not be sufficiently cooled.
Alternatively, the pair of adjacent compression mechanisms may have parallel compressor operation. The suction pressure gas is drawn into both compression mechanisms simultaneously. If, for example, alternative refrigerants are used and the compression mechanisms are in a parallel configuration, the compression mechanisms may be unable to withstand the high operating pressure experienced during compression of some refrigerants such as carbon dioxide.
Additionally, locating the pair of compression mechanisms on opposite ends of the motor requires two drive shafts operatively driven by the motor. The drive shafts have to be precisely aligned and interconnected. The slightest misalignment of the drive shafts will result in dynamic instability. Misalignment of the shafts may also increase the load on the eccentrics, outboard bearings, and main bearings of the compression mechanisms, which in turn will trigger excessive vibration and noise during compressor operation. High pressures and large differences between the suction and discharge pressures will increase the load acting on the drive shafts, which is in turn transferred to bearings. The excess loads may cause premature failure of the bearings.
Some compressors have an eccentric mounted to each end of the shaft being fixedly secured thereto by, e.g., interference fit or a fastener such as a set screw. By providing the compressor with an eccentric that is an independent component from the drive shaft, assembly of the compressor may be complicated. Further, vibration and thus noise may be produced from the eccentric and drive shaft assembly if, for example, the eccentric becomes loose on the shaft.
It is desired to provide a rotary compressor with improved efficiency and reliability having a pair of compression mechanisms located at opposite ends of a drive shaft, operatively driven by the rotor, and improved refrigerant fluid flow through the compressor.